1. Field of the Invention
The present invention relates to a read-out information signal processing circuit, and more particularly to a read-out information signal processing circuit incorporated in an optical recording and reproducing system.
2. Description of the Background Information
In an optical recording and reproducing system using an optical recording medium such as an optical disc and so on, the tracking servo signal may be recorded on the recording medium under the so-called continuous servo format or sampled servo format. The continuous servo format is formed as continuous grooves or lands along the respective tracks. In the sampled servo format, two wobbled pits for tracking and one clock pit for synchronization are formed in each servo area of each track. The wobbled pits and the clock pits are previously formed and are usually referred to as emboss pits or prepits.
FIG. 1 is a block diagram showing a read-out information signal processing circuit in an optical recording and reproducing system for the continuous servo format. In FIG. 1, a read-out information signal (referred to as an RF signal hereinbelow) from a pickup (not shown) is supplied to a head amplifier 1. The RF signal is supplied to a signal processing circuit 3 through a coupling condenser 2 after being amplified by the head amplifier 1. The signal processing circuit 3 performs such a signal processing that equalizes the RF signal to generate a binary signal by slicing the resultant signal at a predetermined level. The output signal of the signal processing circuit 3 is supplied to a decoder 4 for decoding the binary signal. An address and so on including an ID (identifier) signal previously formed as prepits are read out from the output signal of the decoder 4.
In the continuous servo format, data blocks are successively recorded on the recording medium and the RF signal (a) which is read out by the pickup from the recording medium and supplied to the head amplifier 1 has such a waveform as shown in FIG. 2(A). When the recording and reproducing system reads out the ID signal from an ID area of a certain block and irradiates a recording light beam onto a data area succeeding to the particular ID area while the recording light beam is controlled in accordance with a data signal to be recorded. The reflected light from the irradiated position on the disc is received by the pickup, and the RF signal corresponding to the data signal is read out by the signal processing circuit 3. When the recording mode is changed to the reproducing mode, the ID signal and the data signal are read out from the ID area and the data area of a block, respectively. The read-out RF signal during the recording period has a relatively high intensity because the recording light beam is relatively strong. On the other hand, the read-out RF signal during the reproducing mode has relatively low intensity. As a result, the mean level of the RF signal varies stepwise when the operation changes from the recording mode to the reproducing mode. In this instance, it is to be understood that the time constant of the condenser 2 becomes small enough (a cut-off frequency becomes higher than 30 KHz, for example) and it is possible to fully exclude the DC component so that an output signal (b) passed from the condenser 2 has such a waveform as shown in FIG. 2(B). The output signal is compared with the ground level (GND) to produce a binary signal which is then decoded to reproduce the ID signal and the data signal.
FIG. 3 is a block diagram showing a read-out information signal processing circuit in an optical recording and reproducing system adapted for the sampled servo system. An RF signal (c) read out by the pickup has such a waveform as shown in FIG. 4(A). In this case, the read RF signal corresponds to a block without any recorded data, another block with recorded data, and an ID block arranged in sequence along a track. The readout RF signal is supplied through a condesner 11 to an amplifier 12 which amplifies the RF signal. An output signal (d) from the amplifier 12 is supplied to an A/D (Analog to Digital) converting circuit 13. Since the data under the sampled servo format is, for example, 4-15 modulated and the A/D conversion is necessary.
It is necessary that the cut-off frequency by the coupling condenser 11 is determined less than about one tenth of a data sampling frequency (for example, 41.3 KHz) inherent to the sampled servo format so as to accurately reproduce the signal wave. This means that the coupling time constant of the condenser 11 must be relatively large. When, as a result, the read-out RF signal passes through the condenser 11, the DC component of the RF signal cannot be completely eliminated. Thus, the output signal (d) from the amplifier 12 has such a waveform as shown in FIG. 4(B). Namely, a reference level of the read-out RF signal largely deviates from the ground level at the time of the reproducing mode immediately after the end of the recording mode as seen from FIG. 4(B). It is therefore necessary that the A/D converting circuit 13 should have an input dynamic range converting a possible amplitude variation range of the signal which is deviated in the negative direction from the ground level so as to effectively perform the A/D conversion. If, to the contrary, the input dynamic range is relatively narrow, the resolution decreases due to such deviation of the DC level of the amplified RF signal.